This invention concerns a direct current melting furnace with control of deflection of the electric arc:
To be more exact, the invention concerns a system which enables the position of the electric arc to be controlled and conditioned with very great accuracy in a DC. melting furnace according to the requirements which may arise from time to time during a melting operation.
In a DC electric-arc melting furnace the electric arc is generated between an upper electrode in a substantially central position and a lower electrode system located on the hearth of the furnace.
A first type of lower electrode system consists of the so-called conductive hearth, in which the direct current, so as to reach the scrap or molten bath in the furnace, passes through a layer of refractory material, to which is added, or which is replaced by, a conductive material, so that the current is discharged onto a metallic plate and passes thence to the rectifier and transformer.
Another embodiment is the so-called single bar type, in which the current flows from the electrode on the bottom through the molten bath and/or scrap and thence through the arc to the upper electrode.
The embodiment with the conductive hearth entails the problem of affecting the whole hearth of the furnace by passage of the current and requires a continuous control of the temperature over the whole surface involved and cooling means over a very wide surface. Localized wear of the hearth of the furnace may lead to variations in the thickness of the bottom of the same and therefore in the conductivity of the refractory material, thus causing unbalances in the distribution of the current, and these unbalances in turn may lead to deflection of the arc.
This fact necessitates a frequent control of the conditions of the furnace hearth and involves frequent stoppages of processing and high maintenance costs.
The conductors themselves and the metallic masses included in cooperation with the furnace generate distortions which affect the operation and position of the arc.
Moreover, both the above embodiments have the common feature of including one single electrode on the bottom to the furnace, whereby it is not possible to control random and uncontrolled deflections of the arc generally caused by interferences of the magnetic fields in the system of conductors outside the furnace.
Furthermore, these embodiments do not enable the electric arc to be directed towards desired zones inside the furnace when so required by special working conditions.
The state of the art includes melting furnaces which comprise a plurality of bottom electrodes, arranged advantageously in assemblies of two, four or more and acting as lower electrodes.
The state of the art covers also the ability to control these bottom electrodes separately by providing a specific feeder unit comprising at least one transformer and at least one assembly of thyristors for each of the bottom electrodes.
This lay-out, as is disclosed in DE-A-4.035.233, DE-A-4.118,756, EP-A-0225200 and EP-A-0474883, makes possible, by means of individual adjustments, a better control of deflection of the arc which strikes between the upper electrode and bottom electrodes.
The melting furnaces of the state of the art, however, do not overcome satisfactorily the problem of controlling very accurately the vertical positioning of the arc nor the problem of directing the arc towards desired zones of the furnace, because the control is carried out by an open loop control system and because the furnaces do not solve effectively the problem of elimination of interferences caused by the magnetic fields induced by the conductors outside the furnace.
In this connection this latter problem of the state of the art has been tackled with various systems.
A first system, disclosed in EP-A-0258101 for instance, consists in arranging the conductors connected to the bottom electrodes in such a manner that, for instance by using two conductors for each electrode, the magnetic field generated by the first of those conductors affects a well determined zone of the furnace, whereas the magnetic field generated by the other conductor affects the other zone of the furnace, thus endeavouring to balance and cancel the effects of those magnetic fields, but this system is hard to set up and regulate and does not lead to the pre-set results.
A second system, also disclosed in EP-A-0258101, consists in arranging about or below the hearth of the furnace a coil which generates a magnetic field such as will balance, or unbalance in a desired manner, the magnetic fields generated by the conductors, thus cancelling their effects and preventing deflection of the arc, but this system too is hard to set up, does not lead to the pre-set results, entails an increase in energy consumption not justified by the results and restricts considerably in both the upper and lower zones the space available for the work to be carried out on the melting furnace.
Moreover, all the systems intended to minimise disturbances have always been concerned with the bottom electrodes and have never considered the disturbances created by the conductors connected to the upper electrode nor those created by the metallic masses used in the equipment, or in the neighbourhood, of the furnace.